Disentangling the metallicity and star formation history of H ii galaxies through tailor-made models (original) (raw)
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Modeling the ionizing spectra of H II regions: individual stars versus stellar ensembles
Astronomy & Astrophysics, 2010
Aims. We study how IMF sampling affects the ionizing flux and emission line spectra of low mass stellar clusters. Methods. We performed 2 × 10 6 Monte Carlo simulations of zero-age solar-metallicity stellar clusters covering the 20 − 10 6 M ⊙ mass range. We study the distribution of cluster stellar masses, M clus , ionizing fluxes, Q(H 0 ), and effective temperatures, T clus eff . We compute photoionization models that broadly describe the results of the simulations and compare them with photoionization grids. Results. Our main results are: (a) A large number of low mass clusters (80% for M clus = 100 M ⊙ ) are unable to form an H ii region. (b) There are a few overluminous stellar clusters that form H ii regions. These overluminous clusters preserve statistically the mean value of Q(H 0 ) obtained by synthesis models, but the mean value cannot be used as a description of particular clusters. (c) The ionizing continuum of clusters with M clus 10 4 M ⊙ is more accurately described by an individual star with self-consistent effective temperature (T * eff ) and Q(H 0 ) than by the ensemble of stars (or a cluster T clus eff ) produced by synthesis models. (d) Photoionization grids of stellar clusters can not be used to derive the global properties of low mass clusters. Conclusions. Although variations in the upper mass limit, m up , of the IMF would reproduce the effects of IMF sampling, we find that an ad hoc law that relates m up to M clus in the modelling of stellar clusters is useless, since: (a) it does not cover the whole range of possible cases, and (b) the modelling of stellar clusters with an IMF is motivated by the need to derive the global properties of the cluster: however, in clusters affected by sampling effects we have no access to global information of the cluster but only particular information about a few individual stars.
The effects of spatially distributed ionization sources on the temperature structure of H II regions
Monthly Notices of the Royal Astronomical Society, 2007
Spatially resolved studies of star-forming regions show that the assumption of spherical geometry is not realistic in most cases, with a major complication posed by the gas being ionized by multiple non-centrally located stars or star clusters. Geometrical effects including the spatial configuration of ionizing sources affect the temperature and ionization structure of these regions. We try to isolate the effects of multiple non-centrally located stars via the construction of 3D photoionization models using the 3D Monte Carlo photoionization code MOCASSIN with very simple gas density distributions, but various spatial configurations for the ionization sources. Our first aim is to study the resulting temperature structure of the gas and investigate the behaviour of temperature fluctuations within the ionized region. We show that geometry affects the temperature structures in our models differently according to metallicity. For the geometries and stellar populations considered in our study, at intermediate and high metallicities, models with ionizing sources distributed in the full volume, whose Strömgren spheres rarely overlap, show smaller temperature fluctuation than their central ionization counterparts, with fully overlapping concentric Strömgren spheres. The reverse is true at low metallicities. Finally, the true temperature fluctuations due to the stellar distribution (as opposed to the large-scale temperature gradients due to other gas properties) are small in all cases and not a significant cause of error in metallicity studies. Emission-line spectra from H II regions are often used to study the metallicity of star-forming regions, as well as providing a constraint for temperatures and luminosities of the ionizing sources. Empirical metallicity diagnostics must often be calibrated with the aid of photoionization models. However, most studies so far have been carried out by assuming spherical or plane-parallel geometries, with major limitations on allowed gas and dust density distributions and with the spatial distribution of multiple, non-centrally located ionizing sources not being accounted for. We compare integrated emission-line spectra from our models and quantify any systematic errors caused by the simplifying assumption of a single, central location for all ionizing sources. We find that the dependence of the metallicity indicators on the ionization parameter causes a clear bias due to the fact that models with a fully distributed configuration of stars always display lower ionization parameters than their fully concentrated counterparts. The errors found imply that the geometrical distribution of ionization sources may partly account for the large scatter in metallicities derived using model-calibrated empirical methods.
Monthly Notices of the Royal Astronomical Society, 2008
We present a detailed comparison of the ionizing spectral energy distributions (SEDs) predicted by four modern stellar atmosphere codes, TLUSTY, CMFGEN, WM-basic and FASTWIND. We consider three sets of stellar parameters representing a late O-type dwarf (O9.5 V), a mid-O-type (O7 V) dwarf and an early O-type dwarf (O5.5 V). We explore two different possibilities for such a comparison, following what we called evolutionary and observational approaches: in the evolutionary approach, one compares the SEDs of stars defined by the same values of T eff and log g; in the observational approach, the models to be compared do not necessarily have the same T eff and log g, but produce similar H and He I-II optical lines. We find that there is a better agreement, in terms of Q(H 0), the ratio Q(He 0)/Q(H 0) and the shape of the SEDs predicted by the four codes in the spectral range between 13 and 30 eV, when models are compared following the observational approach. However, even in this case, large differences are found at higher energies. We then discuss how the differences in the SEDs may affect the overall properties of surrounding nebulae in terms of temperature and ionization structure. We find that the effect over the nebular temperature is not larger than 300-350 K. Contrarily, the different SEDs produce significantly different nebular ionization structures. This will lead to important consequences on the establishment of the ionization correction factors that are used in the abundance determination of H II regions, as well as in the characterization of the ionizing stellar population from nebular line ratios.
Determination of temperature of the ionizing stars of H II regions
2003
The determination of temperature (T eff) of the ionizing stars of H regions was considered. In this work we used photoionization models for H regions ionized by a single star to show that the index R = log ([O ]λλ3726+3729/[O ]λ5007) can be used to estimate T eff. The relation R vs. T eff proved to be rather independent of the chemical abundances, but strongly dependent on the ionization parameter of the nebula. In order to check the reliability of using R for temperature determination, we compared the values of T eff obtained via the index R for a sample of H regions with data available in the literature with independent estimations.
Monthly Notices of the Royal Astronomical Society, 2011
We analyse the reliability of oxygen abundances and ionization parameters obtained from a number of diagnostic diagrams. To do this, we used the literature to compile the observational emission-line intensities and oxygen abundances of 446 star-forming regions whose O/H abundances were determined by direct estimation of the electron temperature. These compiled abundances were compared with the values calculated in this work using various diagnostic diagrams in combination with results from a grid of photoionization models. We found that the [O III]/[O II] versus [N II]/[O II], [O III]/Hβ versus [N II]/[O II] and ([O III]/Hβ)/([N II]/Hα) versus [S II]/[S III] diagnostic diagrams gave O/H values close to those obtained using the electron temperature, with differences of about 0.04 dex and a dispersion of about 0.3 dex. Similar results were obtained by detailed models, but with a dispersion of 0.08 dex. The origin of the dispersion found with the use of diagnostic diagrams is probably the differences between the real N/O-O/H relation of the sample and the one assumed in the models. This is confirmed by the use of detailed models that do not have a fixed N/O-O/H relation. We found no correlation between the ionization parameter and metallicity for the objects of our sample. We conclude that the combination of two line ratios predicted by photoionization models, one sensitive to the metallicity and the other sensitive to the ionization parameter, which takes into account the physical conditions of star-forming regions, gives O/H estimates close to the values derived using direct detections of electron temperature.
Effective temperature of ionizing stars of extragalactic H ii regions
Monthly Notices of the Royal Astronomical Society
The effective temperature (T eff) of the radiation field of the ionizing star(s) of a large sample of extragalactic H ii regions was estimated using the R=log([O ii](λλ3726+29)/[O iii]λ5007) index. We used a grid of photoionization models to calibrate the T eff-R relation finding that it has a strong dependence with the ionizing parameter while it shows a weak direct dependence with the metallicity (variations in Z imply variations in U) of both the stellar atmosphere of the ionizing star and the gas phase of the H ii region. Since the R index varies slightly with the T eff for values larger than 40 kK, the R index can be used to derive the T eff in the 30 − 40 kK range. A large fraction of the ionization parameter variation is due to differences in the temperature of the ionizing stars and then the use of the (relatively) low T eff dependent S2=[S ii](λλ6717+31)/Hα emission-line ratio to derive the ionization parameter is preferable over others in the literature. We propose linear metallicity dependent relationships between S2 and U. T eff and metallicity estimations for a sample of 865 H ii regions, whose emission-line intensities were compiled from the literature, do not show any T eff-Z correlation. On the other hand it seems to be hints of the presence of an anti-correlation between T eff-U. We found that the majority of the studied H ii regions (∼ 87%) present T eff values in the range between 37 and 40 kK, with an average value of 38.5(±1) kK. We also studied the variation of T eff as a function of the galactocentric distance for 14 spiral galaxies. Our results are in agreement with the idea of the existence of positive T eff gradients along the disk of spiral galaxies.
Line temperatures and elemental abundances in H II galaxies
Monthly Notices of the Royal Astronomical Society, 2003
We present long-slit spectrophotometric observations in the red and near infrared of 12 HII galaxies. The spectral range includes the sulphur lines [SII] at λλ 6716, 6731 A and [SIII] at λ 6312Å and λλ 9069, 9532Å. For all of the observed galaxies, at least three ion-weighted temperatures from forbidden auroral to nebular line ratios have been obtained and the relations between the different line temperatures have been discussed. It is found that, for some objects, the [OII] temperatures derived from those of [OIII] through the use of photo-ionisation models, without taking into account the effect of density, can lead to a significant underestimate of the O + /H + ionic abundance and hence of the total oxygen abundance.
O Star Effective Temperatures and H ii Region Ionization Parameter Gradients in the Galaxy
The Astrophysical Journal, 2004
Extensive photoionization model grids are computed for single-star H ii regions using stellar atmosphere models from the WM-Basic code. Mid-IR emission-line intensities are predicted, and diagnostic diagrams of [Ne iii/ii] and [S iv/iii] excitation ratios are built, taking into account the metallicities of both the star and the H ii region. The diagrams are used in conjunction with Galactic H ii region observations obtained with the Infrared Space Observatory to determine the effective temperature T eA of the exciting O stars and the mean ionization parameterŪ, where T eA andŪ are found to increase and decrease, respectively, with the metallicity of the H ii region represented by the Ne/Ne ratio. No evidence is found for gradients of T eA orŪ with Galactocentric distance R Gal . The observed excitation sequence with R Gal is mainly due to the effect of the metallicity gradient on the spectral ionizing shape, upon which the effect of an increase in T eA with Z is superposed. We show that not taking properly into account the effect of metallicity on the ionizing shape of the stellar atmosphere would lead to an apparent decrease of T eA with Z and an increase of T eA with R Gal .
Star Formation in HII Galaxies. Properties of the ionized gas
2009
We propose a methodology to perform a self-consistent analysis of the physical properties of the emitting gas of HII galaxies adequate to the data that can be obtained with the XXI century technology. This methodology requires the production and calibration of empirical relations between the different line temperatures that should superseed currently used ones based on very simple, and poorly tested, photo-ionization model sequences. Then, these observations are analysed applying a methodology designed to obtain accurate elemental abundances of oxygen, sulphur, nitrogen, neon, argon and iron in the ionsied gas. Four electron temperatures and one electron density are derived from the observed forbidden line ratios using the five-level atom approximation. For our best objects errors of 1% in T([OIII]), 3% in T([OII]) and 5% in T([SIII]) are achieved with a resulting accuracy between 5 and 9% in total oxygen abundances, O/H. These accuracies are expected to improve as better calibratio...